The invention relates to an actuator arrangement for the scanning and/or recording of items of information on recording media. An arrangement, which is provided for fine- or follow-up control or regulation and with which a scanning and/or recording head is positioned in its location with respect to the recording medium, will be referred to as the actuator arrangement. The actuator is a component, in the output section of a control or regulating path, which has a decisive influence on the energy or earth currents and which acts therein as a variable resistance. Actuators are used in, amongst others, optical and magneto-optic recording and reproducing devices such as for example, CD players, in which items of information are scanned from a rotating recording medium, the CD disc, by means of a light beam. An exact positioning of the scanning means is required since even small deviations in the position of the scanning and/or recording head relative to the recording medium lead to losses of information and functional errors. For the reproduction of signals stored on an information carrier, such as for example, pictures and sound in a video disc player, sound in a CD player or data items of a magneto-optic disc, in addition to an exact focusing of the light beam on the disc, a precise guidance along the data track is also required whereby the actuator has to be guided as parallel and as perpendicular to the plane of the disc as possible. The guidance should be smooth and the actuator should exhibit a low mass so that even tiny displacement forces are sufficient for the regulation and a high regulating speed is ensured. On the other hand, the position of the actuator perpendicular to the plane of the disc should be as stable as possible so that vibrations, caused by external influences for example, do not disadvantageously affect the scanning or recording process and the scanning and/or recording head.
A reading head for the reading of information recorded as a track or as tracks on a recording medium which is moved relative thereto, is already known c.f. DE 26 45 326 C2. In this reading head, which has an objective lens for projecting a light point onto the surface of the recording medium, a scanning device for scanning the relative position between the objective lens and the track and for generating a tracking error signal and a focusing error signal, an objective lens carrier which is movable in a first direction parallel to the axis of the objective lens and is attached to the reading head via a compliant support and which is controllable in this first direction by the focusing error signal by means of a focusing coil arrangement and which also has a track following device by means of which the light point is adjustable by the tracking error signal perpendicularly to the objective lens axis and perpendicularly to the track in a second direction, resilient elements are used as the support for the objective lens carrier, which elements are mounted in the form of a helix having the axis of the objective lens as the central axis or symmetrically with respect to the axis of the objective lens and to the second direction. The support for the objective lens carrier is thus also compliant in a second direction perpendicular to the axis of the objective lens and the track following device is constructed, in known manner, as an electromagnetic adjusting device whose movable part is attached to the objective lens carrier and whose fixed part is attached to the reading head. In addition to the track following device, there is provided a further electromagnetic adjusting device whose movable part is attached to the objective lens carrier and whose static part is fixed to the reading head and the resilient elements are so constructed that the support for the objective lens carrier is adjustable by a time base error signal in a third direction perpendicular to the axis of the objective lens and parallel to the track. The support for the objective lens carrier has an intermediate part which is attached to the reading head via a first resilient element that is compliant in a first direction and to which, the movable part of the track following device is attached via a second resilient element that is compliant in the second direction. The first resilient element is formed by leaf springs and the second resilient element by flexible wires which are tensioned between the movable part and the objective lens carrier. In this connection, it was further proposed to use a helical spring which was compliant in every direction as the resilient element or curved spring bars which were compliant in every direction as the resilient elements. The underlying object of developing a reading head of the type referred to, in which the movable parts are constructed to be very light and thus can be very quickly adjusted and in which nevertheless, it is ensured that the axis of the objective lens maintains its spatial orientation insofar as possible, is only inadequately achieved since, in particular, the four resilient wires, on which the tubular objective lens carrier is hung via a holder carrying the coils of the deflecting system and the round plate, which is attached to the base via leaf springs and to which the resilient wires are attached, form a system which is inclined to oscillate and which can be easily excited by the rotating recording medium or external influences. This is likewise applicable for a spiral spring or a helical spring as well as for the three dimensionally curved bars forming a spring. In particular the compliant support of the objective lens carrier by spiral or helical springs is disadvantageous in that suspensions of this type are not symmetrical on all sides so that a deflection in an additional direction is simultaneously associated with its deflection in one direction, which is not desirable. This results from the adjusting force effective on the objective lens carrier not being directly effective but rather being effective via an appropriate magnetic field on the objective lens carrier which is movable relative freely in space and which exhibits further degrees of freedom, so that, contingent upon the asymmetry of the suspension, and in the course of the contactless deflection, a deflection direction is set up that does not coincide with the direction of the deflecting force. It is disadvantageous furthermore, that the actuator arrangement is made up of a multiplicity of parts of different materials whose manufacture and assembly necessitates very careful, mechanically detailed work which represents a high expenditure.
In accordance with the EP-A 0 178 077, an optical scanning device is known whose objective lens is attached to a frame by means of four parallel leaf springs which are deflectable in the direction of the optical axis--perpendicular to the surface of the disc--and which are referred to as focusing springs. The frame is connected to a fixed part of the housing via four further parallel leaf springs whose vibration plane is at right angles to the plane of vibration of the four focusing springs. The vibration plane of the leaf springs referred to as focusing springs lies parallel to the surface of the disc. This actuator arrangement is likewise disadvantageous in that it is made up of a multiplicity of parts of different materials and, due to the use of leaf springs, is readily inclined to oscillate. Eight leaf springs are required in order to enable deflection movements in two directions of vibration. Since the planes of vibration or deflection should be exactly perpendicular to one another, the manufacture and assembly of the parts necessitates high precision.
Deflection movements which are only nearly linear are achieved since the deflection of a spring parallelogram attached to a stationary body leads to a curved movement.
Furthermore, a scanning device is known, c.f. DE 39 24 190 A1, which merely requires two holding devices, as opposed to eight leaf springs, for the realisation of the two directions of deflection. These holding devices are attached at one end to two opposite sides of the scanning device or of a part of the scanning device and the other end is mounted on a fixed holder. Each holding device is built up from two end sections, two intermediate sections and at least one mid section which are connected together by hinges. The parallel axes of rotation of the two outer hinges, which connect the end sections to the intermediate sections, are perpendicular to the parallel axes of rotation of the two inner hinges, which connect the mid section to the intermediate section. The holding device is made in one piece from synthetic material, in which notches at the hinge points form yield points that serve as hinges. Due to the use of parallel guides, actuator movements both perpendicular to as well as parallel to the plane of the disc are made possible. These parallel movements provided for the focusing and the track following are usually adequate for CD players. However, without this entailing an expensive adjustment procedure, the perpendicular and parallel alignment of the actuator arrangement relative to the plane of the disc is crucially determined by the dimensional stability of the sections of synthetic material. Manufacturing tolerances and high ambient temperatures which lead to distortions thus act disadvantageously on the precision of the actuator guidance.
In general, high and substantially differing ambient temperatures such as occur for example, during operation in motor vehicles, have a disadvantageous effect on actuator arrangements which incorporate parallel guides and, due to the parallel guidance, they are not correctable in every direction relative to the recording medium. Internal tensions in actuator arrangements made up of different materials and, for parts of synthetic material, their deformation, lead to changes which, due to distortion or a twisting of the actuator arrangement in particular, manifest themselves as angular deviations.
An actuator arrangement is already known whose spatial positioning with respect to the information carrier can be constantly corrected by means of a manoeuvering device c.f. DE 32 34 288 C2. It is constructed without any mechanical guides and the objective lens carrier is freely suspended in a magnetic field. The objective lens carrier consists of a permanent magnetic sleeve which is provided with magnetic poles and there are two sets of coils each having at least three coils arranged in the outer back-circuit field of the permanent magnetic sleeve. The coil fields at the axial ends of the sleeve thereby exert an effect in three directions which are perpendicular to one another in accordance with the way they are controlled. Furthermore, the coils are positioned in such a way that two torques are generated in an axis perpendicular to the axis of the objective lens. It is disadvantageous that a large air gap is required between the movable actuator and the static parts in order to avoid mechanical contact of the responsive objective lens carrier with fixed parts of the actuator arrangement. The arrangement thus exhibits only a low efficiency and large demands must be placed on the precision of the electromagnetic fields so that deviations resulting from a lack of symmetry do not lead to contacts. Six coils and correspondingly expensive control and regulating devices are needed in order to compose the desired three forces in the direction of the co-ordinate axes and the two torques from the six individual forces. The regulation of the location of the objective lens carrier that is freely suspended in the magnetic field requires a large outlay and the mass of the movable permanent magnetic sleeve that is freely suspended in the magnetic field is disadvantageous in that vibrations lead to an undesired deflection. Moreover, damage to the objective lens carrier can occur during transportation in the off state due to a lack of damping.